# Is there a cost effective way to obtain Nitrogen/Argon on Mars for usage in a human breathable habitat?

Is there a cost effective way to obtain Nitrogen/Argon on Mars for usage in a human breathable habitat?

It is my understanding that a sustainable human breathable atmosphere is made of two major parts, oxygen and inert gasses.

(The most common 2 inert gasses found on mars are Nitrogen and Argon, but an answer that includes any gas obtainable on Mars that is mixable with oxygen to produce a sustainable human breathable atmosphere would be accepted)

P.S. Assume oxygen is available from an alternative solution. How do you make oxygen on mars

• CO2 has a fairly high boiling/melting point compared to O2/N2/Ar. Under pressure, those temperatures shift higher, so if you simply pressurize Mars air, dump the liquid CO2, and pass the gaseous component to further stages, you could significantly enrich the Ar/N2/O2 without much problem. – Nick T Mar 14 '15 at 18:43
• Is there any benefits to liquefying c02 vs freezing it? (Does it take less energy to condense sufficient atmosphere, then to freeze it?) – MarsOneOrBust Mar 14 '15 at 19:26
• It's mechanically simpler to compress a gas to condense it (if you can easily push it through some phase transition) instead of refrigerating it. – Nick T Mar 14 '15 at 19:29
• From my research, mars regolith has very little nitrogen in the ground (at least on the surface). Do you by chance have a source as to where this ground nitrogen can be found? – MarsOneOrBust Mar 15 '15 at 15:49
• Mars atmosphere is 2% nitrogen and 2% argon. Most of the rest is CO2. Freezing out the carbon dioxide to get nitrogen and argon is doable. – HopDavid Mar 25 '15 at 15:05

## 3 Answers

Both argon and nitrogen are usually produced on Earth by freezing a volume of air until it liquefies, and then separating its component chemicals by carefully adjusting the temperature so that each one boils off separately and is collected. This is very energy intensive. The lower temperatures on Mars would mitigate this to some extent, but on the other hand, Mars experiences big temperature swings between day and night. That would probably make temperatures much harder to control in the precise manner needed.

More recently, pressure swing adsorption has begun to be used for producing nitrogen, and for sequestration of carbon dioxide. This works by pumping air through a high-pressure chamber filled with adsorption material like activated carbon or zeolites. There are synthetic zeolites that selectively absorb nitrogen. The quote below is from this link, from the New Zealand Institute of Chemistry:

Zeolites are aluminosilicate minerals with complex crystal structures made up of interlocking rings of silicon, aluminium and oxygen ions. The chemical composition of the zeolite used for oxygen separation is Na12[(AlO2)12((SiO2)12].27H2O. It is the zeolite’s shape which provides most of the ability to selectively adsorb nitrogen. The zeolite used for oxygen production is shaped like a die with holes drilled on each face to form an internal cage. The corners of the die (providing the framework) are SiO2 and AlO2 units. Cations (either Na or Ca) are exposed throughout the crystal lattice. This zeolite is shown in Figure 1.

(It is sometimes used for oxygen production on Earth because removing atmospheric nitrogen leaves mostly oxygen.) The air that exits such a pressure chamber is stripped of some of its nitrogen. Once the zeolite has been fully charged in this way with nitrogen, the pressure in the chamber is reduced, and the nitrogen naturally releases again as a gas, and can be collected. Over several passes, the concentration of nitrogen in the resulting gas is increased, it can in very refined systems be made of high purity.

But for Mars, maybe something like 90% would be enough, in the interest of reducing system power needs, size, and mass that needs to be launched from Earth. The effectiveness of the system depends on how good the zeolites are. In some ways, this is a good thing. They wouldn't have any trouble surviving the rigors of launch and landing, and they work passively, meaning they don't need repairs and maintenance. They can be used over and over, just release the pressure, and they release their nitrogen, then they are ready to adsorb more. On the other hand, they are heavy. I can't find figures so far on how much nitrogen you can expect to get from each cycle from each kilogram of zeolite (density of about $0.7 gm/cm^3$), but they are like porous rock. I bet it isn't much. Perhaps the technique that has been proposed for other resources, of sending ahead an automated plant that will run on its own, storing up nitrogen while it awaits the arrival of the first crew, could work. The mechanical parts in the system are pretty simple - pumps and pressure chambers. It sounds like something that could reasonably be automated, and the hardware parts aren't difficult to repair or replace.

At least the power needs of the system, compared to cryogenic separation, are much lower. Which is a whole other can of worms - how many nuclear thermal generators a crewed base would need, and whether a solar power system could be an effective substitute. At any rate, much of the mass you would have to ship in zeolite could be defended by counting up the mass of power generators, insulators, and heat exchangers you wouldn't need to ship because a cryogenic system isn't being used. This table from the same article linked above, by NZIC, makes clear how much less power PSA requires:

• How similar would a nitrogen capable zeolite be to the co2 zeolite systems (CDRA) already used in a space environment (Skylab, ISS)? (AKA: does such a technique still require lots of research?) – MarsOneOrBust Mar 15 '15 at 17:08
• @MarsOneOrBust i added some detail to help answer that. It is a mature technology, no research needed. The removal of CO2 is a much more challenging job. – kim holder Mar 15 '15 at 19:12
• Also worth mentioning: Vacuum swing adsorption which is a subset of PSA, and even more promising for Mars is Temperature swing adsorption as it can use the temperature difference between night and day, or low quality waste heat, to provide most the required energy. membrane gas seperation may also be useful for seperating nitrogen altough robustness is likely to be a problem. – Blake Walsh Mar 16 '15 at 8:45

Recently, the Curiosity rover on Mars discovered nitrogen in the form of nitrates. Depending on the location and size of the nitrate deposits it may be possible to mine the nitrates as a source of nitrogen.

• The nitrogen in the Mars air is already in the form required. To get nitrates from a mined source, first you pay for the mining, and then for the chemical process to extract gaseous nitrogen from that. It isn't likely to be an economical source of nitrogen. – kim holder Mar 25 '15 at 16:16

All you probably have to do is compress Mars' atmosphere and CO2 will liquefy while N2 and Ar will remain gaseous.